The invention generally relates to vulcanizable elastomeric compounds containing silica as a reinforcing filler.
When producing elastomeric compositions for use in rubber articles, such as tires, power belts, and the like, it is desirable that these elastomeric compositions are easily processable during compounding and have a high molecular weight with a controlled molecular weight distribution, glass transition temperature (Tg) and vinyl content. It is also desirable that reinforcing fillers, such as silica and/or carbon black, be well dispersed throughout the rubber in order to improve various physical properties, such as the compound Mooney viscosity, modulus, tangent delta (tan xcex4), and the like. Rubber articles, especially tires, produced from vulcanized elastomers exhibiting these improved properties will have reduced hysteresis, better rolling resistance, snow and ice traction, wet traction, and improved fuel economy for vehicles equipped with such tires.
With the increasing use of silica as a reinforcing filler for rubber, filler dispersion in rubber stocks has become a major concern. Because polar silanol groups on the surface of silica particles tend to self-associate, reagglomeration of silica particles can occur after compounding, leading to poor silica dispersion and a high compound viscosity. The strong silica filler network results in a rigid uncured compound that is difficult to process in extrusion and forming operations. Previous attempts at preparing readily processable, vulcanizable silica-filled rubber stocks containing natural rubber or diene polymer and copolymer elastomers have focused on the use, during compounding, of bifunctional silica coupling agents having a moiety (e.g., a silyl group) reactive with the silica surface, and a moiety (e.g., a mercapto, amino, vinyl, epoxy or sulfur group) that binds to the elastomer. Well known examples of such silica coupling agents are mercaptosilanes and bis(trialkoxysilylorgano) polysulfides, such as bis(3-triethoxysilylpropyl) tetrasulfide which is sold commercially as Si69 by Degussa.
In particular, xcex3-mercaptoalkyltrialkoxysilanes have been reported to offer excellent coupling between rubber and silica, resulting in rubbers having improved wet ice skid resistance, rolling resistance and tread wear, as well as improved adhesion between rubber and cords. However, the high chemical reactivity of the xe2x80x94SH functions of the mercaptosilanes with organic polymers can lead to unacceptably high viscosities during processing and to premature curing (scorch). The tendency of a rubber compound to scorch makes compounding and processing more difficult. Mixing and milling must be done more quickly, yet at lower temperatures (e.g., 120xc2x0 C. to 145xc2x0 C.), so that the compound will not begin to vulcanize before it is shaped or molded. The low processing temperature results in a marked reduction in the mechanical activity of mixing which is essential for an optimum dispersion of the silica throughout the polymer matrix. Therefore, compared with carbon black-filled compositions, tread compounds having good silica dispersion require a longer mixing time at a lower temperature to achieve improved performance, resulting in decreased production and increased expense. Moreover, like bis(trialkoxysilylorgano) polysulfide silica coupling agents, mercaptosilanes are relatively costly.
Another disadvantage of the use of mercaptosilane silica coupling agents is that low temperature mixing results in a relatively slow rate of the chemical reaction between the alkoxysilyl portion of the mercaptosilane and the silica (the alkoxysilane-silica reaction). Because this reaction results in the release of a substantial amount of alcohol, a slow reaction rate results in the presence of unreacted alkoxysilyl groups in the compounded product that are then available to further react with the silica and moisture during storage, extrusion, tire build, and/or curing, resulting in an undesirable increase in the compound viscosity, and a shorter shelf life. Moreover, the continuing reaction in the compound evolves more alcohol, resulting in porous zones or blisters which can form surface defects in the resulting formed rubber articles and/or can impair the dimensional stability of treads during extrusion and tire building. As a result, a low tread strip drawing speed must be maintained to ensure that the drawn product conforms with specifications, resulting in a further decrease in production and concomitant increase in costs.
To address the expense and other problems related to mercaptosilanes and other bifunctional silica coupling agents, recent approaches to improving dispersion of silica in rubber compounds have been directed to reducing or replacing the use of such silica coupling agents by employing silica dispersing agents, such as monofunctional silica shielding agents (e.g., silica hydrophobating agents that chemically react with the surface silanol groups on the silica particles but are not reactive with the elastomer) and agents which physically shield the silanol groups, to prevent reagglomeration (flocculation) of the silica particles after compounding. For example, silica dispersing agents, such as alkyl alkoxysilanes, glycols (e.g., diethylene glycol or polyethylene glycol), fatty acid esters of hydrogenated and non-hydrogenated C5 and C6 sugars (e.g., sorbitan oleates, and the like), polyoxyethylene derivatives of the fatty acid esters, and fillers such as mica, talc, urea, clay, sodium sulfate, and the like, are the subjects of EP 890603 and EP 890606. Such silica dispersing agents can be used to replace all or part of expensive bifunctional silica coupling agents, while improving the processability of silica-filled rubber compounds by reducing the compound viscosity, increasing the scorch time, and reducing silica reagglomeration. To achieve a satisfactory cure of the rubber compound, the use of silica dispersing aids includes employing an increased amount of sulfur in a mixing step when curing agents are added to the composition, to replace sulfur that otherwise would have been supplied by a sulfur-containing silica coupling agent.
An advantage of the use of silica dispersing aids during compounding of elastomers with silica is that, unlike the bifunctional silica coupling agents described above, the dispersing agents do not contain sulfur and, thus, they can be used at high temperature, e.g., about 165xc2x0 C. to about 200xc2x0 C., in the absence of curing agents, without increasing the risk of premature curing. At these high temperatures, the reaction between the silica and alkoxysilyl groups of alkyl alkoxysilane silica dispersing agents is accelerated, resulting in an increase in the amount of alcohol evolved and evaporated during compounding, and a decrease in evolution of alcohol from the compound during storage, extrusion, curing and tire build.
Unexpectedly, it has been discovered that improvements in the tensile mechanical properties and dynamic viscoelastic properties of silica-reinforced sulfur vulcanized rubbers can be achieved by compounding polymers with silica at a temperature of about 130xc2x0 C. to about 200xc2x0 C., in the presence of an alkyl alkoxysilane silica dispersing aid and a very small amount of a mercaptosilane silica coupling agent. The terms elastomer, polymer and rubber are used interchangeably herein, as is customary in the rubber industry. In particular, the weight ratio of the mercaptosilane to the alkyl alkoxysilane is a maximum of 0.14:1, preferably about 0.001:1 to about 0.10:1, and typically about 0.01:1 to about 0.10:1. The mercaptosilane is present in the compound in an amount of about 0.0001% to about 3% by weight, typically about 0.001% to about 1.5% by weight, and especially about 0.01% to about 1% by weight, based on the weight of the silica. It has been discovered that the use of such a small amount of the mercaptosilane, even at a high mixing temperature, unexpectedly does not result in premature curing. Therefore, the mercaptosilane and alkyl alkoxysilane can be mixed with the elastomer and silica reinforcing filler in the first stage of the mixing process, at a higher temperature (e.g., about 155xc2x0 C. to about 200xc2x0 C., especially about 170xc2x0 C. to about 185xc2x0 C.) than previously allowable for conventional amounts of mercaptosilane coupling agents, allowing a shorter mixing time with a concomitant savings in production time and expense, and improved performance of the ultimate rubber product.
In the compounding of the elastomer with silica, the mercaptosilane facilitates binding of the silica by the polymer, and the alkyl alkoxysilane provides a desirable compound viscosity for processability, resulting in vulcanized elastomeric compounds demonstrating tensile mechanical and dynamic viscoelastic properties that are improved over similar compounds prepared with alkyl alkoxysilanes at the same temperature, in the absence of the mercaptosilane. These properties of the invention compounds also are comparable to, or improved over, similar compounds prepared with conventional amounts of bis(trialkoxysilylorgano) polysulfide silica coupling agents, such as Si69, at about 160xc2x0 C. or less. A conventional amount of Si69, for example, is about 5% to about 20% by weight, based on the weight of the silica.
In particular, the vulcanized elastomeric compounds of the invention exhibit improved compound viscosity, improved dispersion of silica, reduced filler flocculation after compounding, increased bound rubber content, and decreased evolution of alcohol during extrusion, curing, and tire build, resulting in lower hysteresis and improved wear resistance in the vulcanized product. The compounds also exhibit improved dynamic viscoelastic properties, especially a higher tensile modulus at 300% strain, a lower storage modulus (Gxe2x80x2) at xe2x88x9220xc2x0 C., a higher tan xcex4 at 0xc2x0 C., and a lower tan xcex4 at 50xc2x0 C. Such properties have been commonly used in the tire industry to predict tire performance in the categories of snow and ice traction (Gxe2x80x2 at xe2x88x9220xc2x0 C.), wet traction (tan xcex4 at 0xc2x0 C.), and rolling resistance (tan xcex4 at 50xc2x0 C.).
The invention also provides a method for making the sulfur vulcanized compound of the invention, and a pneumatic tire having at least one component that comprises the vulcanized compound.